Excellent question!
I pondered this question a while before answering it as I've never really thought about it before. IMO it likely stems from "old school sportsman". I can certainly understand why is would be considered "poor sport" for shooting a bird on the ground or water. It essentially takes 0 skill to shoot a bird with bird shot that isn't moving. It increases the odds however and makes sence that it should be the "ethical shot". I guess in this sence it's not "sporting" to take the "ethical"shot.

For big game and turkey it is much harder to get close to the animal. I agree with Silent that there is more "vaule" put on the life of a big game animal, not that I agree that this is how it should be. I think that the sport in shooting an unexpecting big game animal comes for getting the opportunity at a clean shot. Wether or not you choose to shoot the animal is second to getting a good high percentage clean shot. This is were the sport is as well as it being the ethical shot.

As I see it the diffrence between the two is where sportsmanship and ethics overlap. With birds the ethical shot dosn't overlap with the ideals of the sportsman and with turkey and big game there is a clear overlap between what is considered good sportsmanship and what is ethical. Why this is is the million dollar question though.